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Two manganese oxide phases, α-Na_0.51Mn_0.93O₂ and β-Na_0.52MnO₂, have been synthesized and their structures determined by Rietveld analysis: α-Na_0.51Mn_0.93O₂ (isostructural to NaCo₂O₄) space group P6₃/mmc, a = 2.8661(4) Å, c = 11.165(4) Å and β-Na_0.52MnO₂ (isostructural to α-K_0.51Mn_0.93O₂), space group Cmcm, a = 2.834(6) Å, b = 5.216(1) Å and c = 11.280(3) Å. Both phases are layered, with alternate sheets comprised of (MnO₆) octahedra and (NaO₆) trigonal prisms. Ion exchange of Na for Li in a sample having the mean composition Na_0.56Mn_0.86O₂ that represents a 1∶3 mixture of the α- and β-phases, was effected by immersion in an LiCl/LiNO₃ eutectic to obtain the lithiated composition Li_0.54Na_0.02Mn_0.86O₂. The latter material displayed excellent electrochemical behavior when employed as a cathode in a lithium battery. A high and relatively stable capacity of 160 mA g⁻¹ was obtained in the 2.5–4.3 V voltage window for a C/15 regime. No transition to a spinel structure occurred, on cycling: the discharge/charge curves showed only a monotonous potential variation even after 50 cycles.

The synthesis of orthorhombic LiMnO2 (O-LiMnO2) with very small crystals (diameter≈0.3 m) leads to peculiar X-ray diffraction patterns. Some reflections (with k even) remain thin allowing for cell parameter refinements, showing that, compared to phases with bigger crystals, b and c remain unchanged, whereas an important increase of the a parameter is observed. Other reflections (with k=2n+1 and h≠0) are widened substantially, while the remnant peaks (k=2n+1 and h=0) undergo a strong asymmetrization. These features have been related successfully to faults corresponding to a b/2 translation of a basic unit constituting O-LiMnO2 . A simulation made with the Diffax program allowed good reproduction of the experimental X-ray diffraction data, showing a statistic distribution of the faults, at least for the low fault concentrations corresponding to the samples under study. The insertion of the fault corresponds to the insertion of a monoclinic cell between two blocks of orthorhombic symmetry. This cell (a≈5.53 Å, b≈2.80 Å, c≈5.30 Å) corresponds to a newly obtained monoclinic LiMnO2 phase obtained through a topotactic deintercalation of α-NaMnO2 . The fault percentage of the compounds studied goes from 1 to 6% and is well correlated to the substitution ratio between lithium and manganese when the fault occurrence is treated as a cationic disorder (only in the case of small disorder for which the lines remain treatable with the Rietveld refinement program). The fault percentage can also be determined easily from the cell parameter relation a=x0a0+x mamsinγ, where a0 and am are the parameters of the orthorhombic and monoclinic cell of the pure phases and x0 and xm their relative fractions, a being the parameter of the faulted phase as refined from the fault-unaffected thin reflection peaks.

We have followed the evolution of the superconductingTc on intercalation of Li in ZrSe2 by means of magnetization studies at very low temperatures. To interpret the electronic structure and instabilities, we present the results of extensive LMTO-ASA band structure calculations on ZrSe2 and the idealized compound LiZrSe2 . The experimental results suggest a picture of phase segregation in the intercalated samples into regions where Li atoms are a part of the percolating cluster and regions where the Li are not. The electronic structure calculations help to understand the nature of the bonding in this system and in particular, the nature of instabilities on the Fermi surface in the intercalated compounds. In keeping with certain models for the occurrence of superconductivity in systems whose electronic structures combine localized and itinerant behaviour, the title compounds display regions of high and low dispersion in the Fermi surface. A comparison with the superconducting spinel LiTi2O4 is particularly fruitful.